oooh - tough question. My husband and I (both 30 w/ no kids) toured Universal for 3 days in April this year - we loved both parks.
Since we are coaster enthusiasts, I'd have to say that IOA was the BEST park overall. In fact, its our favorite park of all. Of course, seeing IOA means missing Men in Black at Universal - our favorite ride! I guess I'm not much help, since we loved both parks.
If you don't like coasters, though, I'd probably say Universal, since they don't have the traditional coasters.
Whichever you choose, I'm sure you'll have a wonderful time. BTW, are you staying onsite? We HIGHLY recommend it - best money we ever spent at a theme park with FOTL access ALL DAY every day!!!